Fix SigV4 canonical path handling for escaped URLs

[ryanfowler]

Summary

• Build the AWS SigV4 canonical URI from URL.EscapedPath() instead of the decoded URL.Path
• Preserve escaped octets like %2F and normalize percent-encoding case for canonical signing
• Remove the obsolete decoded-path escaping helper and add coverage for escaped slash, escaped space, and non-ASCII segments

Testing

• go test -v ./internal/aws
• staticcheck ./...
• go test -v ./...